Microbiology Dr. Jawad Kadhim Tarrad
Gram-negative rods related to respiratory tract:
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Three bacterial genera of Gram-negative rods are important pathogenic for respiratory tract of human.
(I) Bordetella:
Important properties:
• Small gram-negative cocobacilli .
• Non-motile.
• Encapsulated rod.
• It is aerobe.
• It is unable to survive outside its host for long time.
Most cases 95% of whooping cough are caused by B.pertussis, whereas 5% of cases are caused by B.parapertussis.
Transmission;
It is found in nasopharynx and respiratory tract mucosa. Transmission is by direct contact , by droplets from respiratory tract , and by freshly contaminated fomites. Most cases occur in children under age 5 years, and most deaths occur in first year of life. It is highly communicable and cause highly morbidity and mortality.
Antigenic structures and virulence factors;
1. Attachment of organism to ciliated epithelial cells of URT is mediated by protein on pili called filamentaous hemagglutinin (Fha).
2. Pertussis toxin(Ptx): consist of 2 chains ,A(active subunit) and B(binding subunit); A single subunit , B composed of 5 non-identical subunits. Ptx has two effects , one is stimulate adenlylate cyclaes in manner of cholera toxin. This result in edema and other changes in respiratory tract, leading to cough. Second effect also cause lymphocytosis in blood by inhibits signal transduction , resulting in failure of lymphocytes to enter lymphoid tissue(spleen and lymph nodes).
3. Cytolysin is bacterial adenylate cyclase toxin that can inhibits bactericidal activity of phagocytic cells.
4. Tracheal cytotoxin is act in concert with endotoxin to kills the ciliated epithelial cells.
Pathogenesis and clinical findings;
The organism adheres to and multiplies rapidly on ciliated epithelial surface of trachea and bronchi and interferes with ciliary action. The blood is not invaded. The bacteria liberate toxins and substances that irritate surface cells, causing coughing and marked lymphocytosis. Necrosis of part of epithelia may occur. The organism is cause ciliotasis (damage ciliated cells of respiratory tract). Decreased cilia activity followed by death of ciliated epithelial cells is important aspects of pathogenesis. The combined action of at least three toxins inhibits normal respiratory clearance mechanisms. Obstruction of small bronchioles by mucus plugs results in atelectasis and diminished oxygenation of blood. This problem contributes too frequently of convulsions in infants with whooping cough.
Clinical significance:
B.pertussis is obligate human parasite (infect only human). The incubation period of pertussis (whooping cough) is ranged from 1-3 weeks. The whooping cough is acute tracheobronchitis can be divided in the following stages;
• Catarrhal phase; it begins with nonspecific symptoms(flu-like) such as rhinorrhea(runny nose),malaise, low fever and then progresses to mild dry ,nonproductive cough and sneezing. Large numbers of organism are sprayed in droplets, and highly infectious.
• Paroxysmal phase; it is characterized by severe coughing followed inspiratory whoop. Large amount of mucus may be produced. The disease gets its name from sharp intake of air between coughing paroxysms that sounds like a whoop. Leukocytosis is occurring in this phase. Paroxysms may cause cyanosis and /or end with vomiting.
• Convalescence phase; it requires at least additional 3-4 weeks. During this period, secondary infection may occur such as pneumonia, otitis media or seizures.
N.B:
Several agents such as adenoviruses and Chlamydia pneumoniae can produce clinical pictures resembling to whooping.
Lab.Diagnosis:
1. Saline nasal wash is preferred specimen. Nasopharyngeal swabs or cough droplets expelled onto cough plate.
2. Culture on Bordet-Gengou medium , the initial isolation usually require to 10% CO2 to enhances the growth of the isolates.
3. Serologic tests have little diagnostic help because arise antibodies does not occur until third week of illness. Agglutination test, fluorescent antibody test are used in diagnosis of this organism.
Treatment:
1. Drug of choice is erythromycin, it is used to reduce number of organism.
2. Oxygen therapy and aspiration of respiratory mucus are important especially in infants.
3. Two pertussis vaccines are available;
(i)Acellular vaccine containing purified proteins ( such as Fha, ptx) is combined with toxoid of diphtheria and tetanus (triple vaccine; DTP).
(ii)Killed vaccine containing inactivated organism.